Even after twenty years we 
still do not know how the 
inputs to cortical cells are 
wired in order to bring about 
this behavior. Several plausible 
circuits have been proposed, 
and it may well be that one of 
them, or several in 
combination, will turn out to 
be correct. Simple cells must be 
built up from the antecedent 
cells with circular fields; by far 
the simplest proposal is that a 
simple cell receives direct 
excitatory input from many 
cells at the previous stage, cells 
whose receptive-field centers 
are distributed along a line in 
the visual field, as shown in 
the diagram to the left.

This type of wiring could 
produce a simple-cell receptive 
field. On the right, four cells 
are shown making excitatory 
synaptic connections with a 
cell of higher order. Each of the 
lower-order cells has a radially 
symmetric receptive field with 
on-center and off-surround, 
illustrated by the left side of 
the diagram. The centers of 
these fields lie along a line. If 
we suppose that many more 
than four center-surround 
cells are connected with the 
simple cell, all with their field 
centers overlapped along this 
line, the receptive field of the 
simple cell will consist of a 
long, narrow excitatory region 
with inhibitory flanks. 
Avoiding receptive-field 
terminology, we can say that 
stimulating with a small spot 
anywhere in this long, narrow 
rectangle will strongly activate 
one or a few of the center-
surround cells and in turn 
excite the simple cell, 
although only weakly. 
Stimulating with a long, 
narrow slit will activate all the 
center-surround cells, 
producing a strong response in 
the simple cell.

    It seems slightly more 
difficult to wire up a cell that is 
selectively responsive to edges, 
as shown in the third drawing 
(c) in the diagram at the head of 
the section. One workable 
scheme would be to have the 
cell receive inputs from two 
sets of antecedent cells having 
their field centers arranged on 
opposite sides of a line, on-
center cells on one side, off-
center cells on the other, all 
making excitatory connections. 
In all these proposed circuits, 
excitatory input from an off-
center cell is logically 
equivalent to inhibitory input 
from an on-center cell, 
provided we assume that the 
off-center cell is spontaneously 
active.

    Working out the exact 
mechanism for building up 
simple cells will not be easy. 
For any one cell we need to 
know what kinds of cells feed 
in information--for example, 
the details of their receptive 
fields, including position, 
orientation if any, and whether 
on or off center--and whether 
they supply excitation or 
inhibition to the cell. Because 
methods for obtaining this kind 
of knowledge don't yet exist, we 
are forced to use less direct 
approaches, with 
correspondingly higher 
chances of being wrong. The 
mechanism summarized in the 
diagram at left seems to me the 
most likely because it is the 
most simple.